1. Field of the Invention:
The present invention relates to pluri-tubular aerators suitable for use in the modification of the quality of water in reservoirs such as lakes, ponds or dams, or in harbors.
2. Description of the Prior Art:
In the modification of the quality of water in a lake, for example, it has been customary practice to interchange the surface water of a high dissolved oxygen content with the bottom water of a low dissolved oxygen content for activating the bottom of the lake, thereby purifying the water in the lake.
In order to circulate the lake water, there have been proposed two types of aerators; one being a continuous aerator, the other being an intermittent aerator.
A typical example of known continuous aerator, as shown in FIG. 11 of the accompanying drawings, includes a single uptake tube 1 disposed vertically on the bottom of a lake, and a diffuser 2 disposed in the uptake tube 1 adjacent to the lower end thereof.
Upon receipt of a supply of compressed air, the diffuser 2 produces a stream of fine air bubbles 3 rising in and along the uptake tube 1. In this instance, a large amount of oxygen is dissolved into the water, and due to the rising air bubbles 3, water is moved upwardly at a constant flow velocity, as shown in FIG. 12, circulating throughout the depth of the lake. With this circulation, the dissolved oxygen content in the bottom water is substantially increased with the result being that the bottom of the lake is activated to thereby prevent dissolution of iron, manganese, phosphorus, nitrogen or the like from the bottom of the lake, and also enable inhabitation of aquatic life.
The continuous aerator of the foregoing construction is advantageous in that an increased amount of dissolved oxygen is obtained. This aerator however has a drawback in that the effective aeration area is narrow and sufficient circulation and agitation of lake water is difficult to obtain.
One example of intermittent aerator, as shown in FIG. 13, includes an uptake tube 4 and a tank 5 disposed at a lower portion of the uptake tube 4 for intermittently producing bulky air bubbles 6 rising in and along the uptake tube 4 for lifting water layers disposed between adjacent bulky air bubbles 6 (Japanese Utility Model Laid-open Publication No. 58-137900, for example).
With this construction, water sealed between adjacent air bubbles is drafted or lifted reliably at a high speed so that as shown in FIG. 14, water is continuously drafted due to inertia even after the arrival of a bulky air bubble at the surface of the lake. As a result, the lift of water relative to the supply of air is greater than that of the continuous aerator. The intermittent aerator of this type however has a drawback in that only a limited effect is obtained for increasing the dissolved oxygen content.
The lift of water per unit air supply varies with the cycle of production of the bulky air bubbles which is determined by the air supply per unit time. It is therefore desirable to select the capacity of a compressor such that compressed air is supplied to the tank at maximum efficiency. In practice, however, when an optimum air supply per unit time is to be 0.7 m.sup.3 /min, a compressor having a capacity of 7.5 KW (0.84 m.sup.3 /min) is generally employed in view of allowance, rather than a compressor having a capacity of 5.5 KW (0.63 m.sup.3 /min).
With this oversized compressor, a subsequent bulky air bubble is produced even when water is still rising under inertia, and hence the use of the inertial force is substantially limited as shown in FIG. 15. Consequently, despite a slight increase of the lift of water obtained, the lift of water relative to the amount of air supply decreases conversely and hence the efficiency of the aerator is lowered as a whole.
The foregoing intermittent aerator having a single uptake tube is relatively small in size and hence ten or more of such small aerators are used for a sufficient circulation of water when the pondage of a lake to be treated is relatively large. However, a lake having a pondage greater than 8 million ton requires a number of such single uptake tube aerators. This system is expensive and hence the small-sized single uptake tube aerator is not used so widely.
With the foregoing difficulties in view, various attempts have been taken to increase the capacity of the conventional intermittent uptake tube aerator, which capacity has been limited by the size of an air bubble formed under water. It has been acknowledged that the maximum diameter of the air bubbles is limited to 500 mm-600 mm and when in an uptake tube having an inside diameter greater than this maximum bubble diameter, produced bubbles are separated into several bulky air bubbles. Such separated air bubbles are no longer effective to seal the inside of the uptake tube and hence the lifting ability is substantially reduced.
According to one proposal disclosed in Japanese Utility Model Laid-open Publication No. 60-176300, there is provided an aerator which includes, as reillustrated here in FIGS. 16A and 16B, a plurality of tubes having an inside diameter of 500 mm-600 mm for providing a corresponding number of maximum air bubbles to thereby increase the lift of water.
The proposed aerator is called a bundle type and is composed of a lower tube 7 having a tank 8 for producing bulky air bubbles 9 (only one shown in FIG. 16A), and four uptake tubes 10 disposed on the top of the lower tube 7. In operation, compressed air supplied by an external source is introduced through an inlet 8a into the tank 8. The compressed air fills the tank 2 soon and then is drawn into the interior of the lower tube 7 instantaneously under siphonage, thereby producing a single bulky air bubble 9. The air bubble 9 then rises in and along the lower tube 7 and upon its arrival at the uptake tubes 10, the air bubble 9 is divided into four air bubbles 9' which in turn separate inside water into upper and lower parts and move these water parts upwardly as they rise in and along the uptake tubes.
Thus, the bottom water in the lake is intermittently lifted by means of the buoyancy of the air bubbles.
Since the lower tube 7 of the foregoing large-sized intermittent uptake tube aerator has a large inside diameter such as 1 m, for example, the air bubble 9 produced therein tends to be separated into a plurality of small bubbles due to the effect of the surface tension and the buoyancy. In this case, the air bubble 9 is not delivered evenly into the four uptake tubes 10 and the air bubbles thus distributed are not uniform in size. With this. non-uniformity, an undersized air bubble 9' fails to separate the inside water into upper and lower parts for sealing the corresponding uptake tube 10 with the result being that the lift of water is substantially lowered. As is apparent from the foregoing description, multiplying the number of uptake tubes does not necessarily bring about a corresponding increase in the lift of water. With this difficulty, an intermittent uptake tube aerator of a large capacity has not been realized.